The Moorehead Apartments, a 32-unit multifamily structure in Crystal Beach, Texas, presented all the usual challenges of a mid-rise project for engineers at Aran + Franklin, with several notable twists. Situated on a little peninsula northeast of Galveston, Texas, the structure needed to be elevated above base flood levels, stand up to high Gulf Coast winds, and provide durable performance in a caustic marine environment. Plus, the initial wood-framed structure was designed with two stories, but the owner intends to build out the attic at a future date — moving the project above and beyond typical apartment design.
And of course, budget was tight. The building owner, Morris Moorehead, wanted to keep the project economical, easy to build, and structurally sound, despite the requirements of high wind and coastal water exposure. So he turned to Robert Dinjar, P.E., with Aran + Franklin, who helped him save money, not just in material and construction costs, but in flood insurance as well.
Stock beam simplicity
Dinjar said the biggest revelation for the project came from his decision to standardize the treated glulam beam widths. He recalled that as he was going through his beam calculations for the 60- by 120-foot structure, he realized he had a mix of 3-1/2 and 5-1/4-inch-wide beams.
“As an engineer, I don’t typically cost out individual framing members,” he said. “We just recommend what’s needed to meet minimal structural requirements. But it occurred to me that there might be a better way to do this.”
Dinjar called his local stock beam supplier to ask about pricing and lead times, and got some sage advice in the process. “The treated glulam rep told me it’s often better to go deeper than wider, and the narrower beams are often less expensive,” he said. “I never thought about it that way, but I soon realized that I could save Mr. Moorehead money by replacing many of the 5-1/4-inch-wide glulams with 3-1/2-inch-wide, slightly deeper beams. The 3-1/2-inch treated glulam beams, manufactured by Anthony Forest Products, cost less and are readily available. Plus, I knew the standardized treated beam widths would also simplify construction. In the end, I estimate that it saved him about 10 percent.”
Additional savings add up
Initially, Moorehead presented Dinjar with a generic building plan, but the foundation wasn’t originally designed to be elevated, which is a requirement in Crystal Beach. “Every structure in this area has to be raised at or above base flood elevation,” Dinjar said. “This meant we had to have the first inhabitable level elevated by at least 17 feet above sea level.”
The National Flood Insurance Program allows building owners to earn substantial insurance discounts for every foot the structure is raised above the minimum. “The savings increase for every foot up to four,” said Dinjar. “So I automatically took this structure 4 feet above the minimum required. The natural grade of the property is at 7 feet 2 inches, so I set the ultimate length of the piers above ground at 13 feet 10 inches,” maximizing the owner’s annual insurance discount and saving him thousands of dollars each year.
Designed for future expansion
Beachfront property is at a premium, so smart developers want to make the most of their investment. For example, Moorehead plans to install a pool below the building. This complicated the grid spacing, but the elevated structure provided enough clearance. Treated 12-inch by 12-inch southern pine timber was used for the piling, with final grid spacing for the glulam girders at slightly less than 12 feet by 12 feet.
The owner intends to eventually finish the attic space to add additional leasable units. So Dinjar designed the gable-roofed structure, which had a 7:12 pitch, to allow for future loading. He also needed to leave the attic space open with minimal bracing for the eventual build-out.
Dinjar admited that it was an unusual choice to have a tall gabled roof in a high wind area. “It was not ideal,” he said. “A simple hip roof would have had very different loading, but we made the original design work. Normally the ridge would consist of a single 2×8, braced every 4 feet, but that’s a lot of bracing. So we increased the size of the ridge to use two 2x12s instead, and minimized the bracing to keep the attic area as open as possible. The gable itself goes across the entire building, about 60 feet across with 15 feet of clear height to the ridge. So now the owner has plenty of room to expand that third level when he wants.”
To meet coastal wind requirements, Dinjar specified an 18-gauge strap to tie the double 2x12s to resist uplift; he also specified a collar tie, “even though it was not required, but I think it makes for a better lateral connection.” Rafters are 2×6, 18 feet long, but because of the slope and select bracing locations, they only span about 12 feet.
Engineering the tall gabled roof was certainly a challenge for the high wind area, Dinjar said. “That huge gable will catch the wind, and the additional shear loads added up. By the time you get to the bottom level, there’s a lot of potential overturning forces. So the future attic expansion resulted in some additional costs on the lower part of the structure.”
Dinjar specified wood structural sheathing throughout most of the interior to transfer lateral loads to the glulam beams below. The exterior was completely sheathed with wood structural panels to fully enclose the diaphragm.
“Where we have panel butts, we required that they add a block in the wall cavity,” Dinjar said. “That way, there is no open seam of any kind, and the block allows the framers to get the proper edge nailing on the panel.”
Protection from the elements
The contractor used a proprietary product for the bottom level floor joists, which combines an open web floor truss with a trimmable, wooden I-joist on each end. The joists are enclosed in a soffit, but the glulam structure is open to the elements from below. Because the shoreline is such a corrosive, moist environment, the stock glulam beams used for the ground floor structure were preservative-treated. In addition, according to code, every fastener used on the exterior was specified as hot dip galvanized or stainless. Dinjar designed the structure under 2009 IBC but said the state of Texas still uses 2006 IBC for wind loading, so it meets the requirements for 130-mph, 3-second gusts, exposure C.
Above and beyond
Sound coastal engineering often requires “above and beyond” thinking. Dinjar said that besides opting for the deeper glulams, he also specified more substantive rafter connectors.
“I normally over-specify on things like that,” he said. “The roof is usually the first thing to go in a wind event; roof failure due to uplift is common. I also go beyond the standard 2×4 exterior wall, primarily for insulation. Not many people know that Texas winters can get miserably cold, especially at the coast. And finally, I usually recommend 16-inch spacing for the floor joists, to keep the floors from bouncing.
“Ultimately, an engineer’s goal is to make the structure safe,” he said. “But when I met Mr. Moorehead, he let me know that cost was also an issue, and I wanted to help him build this project. My discovery that he could save money by going with deeper glulam beams was a big one, and I think it will change the way in which I engineer projects in the future. If I have to specify, I’m going to specify the safe but cheaper option for my client, every time.”
Tom Skaggs, Ph.D., P.E., is with APA-The Engineered Wood Association (www.apawood.org), Tacoma, Wash.